Data transmission over wireless networks has evolved considerably in the past decade. Third generation wireless technologies such as the CDMA2000 1xEV-DO standard enable wireless voice transmission over data networks, such as voice over Internet Protocol (VOIP).
One possible implementation of a mobile voice over data application is a push to talk application (PTT) over 1xEV-DO. EV-DO originally stood for “evolution data-only,” but has come to be known as evolution data-optimized, since technologies like VOIP make it possible to transmit voice over data links. PTT applications, in general, are popular for enabling communications with groups of mobile units, since voice messages to groups can be carried out with a single call. PTT applications that transform mobile phones into “walkie-talkies” are also popular with regular mobile telephone users, but the ability to transmit messages to groups has made push to talk applications particularly useful in setting up dispatch systems. Dispatch systems make good candidates for VOIP PTT systems since fleets of vehicles may be efficiently managed by push to talk dispatch systems operating over secured Internet links.
One constraint on the user-friendliness and widespread adoption of mobile phone PTT systems is the amount of time required to setup a connection. While some currently deployed systems boast connection times under one second, the actual delay between a user's pressing of the “PTT” button on their mobile device and their receipt of the talk permit tone indicating connection (the “push to beep” time or PTB time) is still long enough to present a noticeable and undesirable delay to users. The talk permit tone is applied after both originating and terminating users have acquired the traffic channel.
The current connection setup protocol defined in the 1xEV-DO standard is a serial process. The 1xEV-DO standard is defined in the Telecommunications Industry Association's standards such as (equivalent free of charge standards from the 3rd Generation Partnership Project 2 industry consortium in brackets): TIA-856-A (3GPP2 C.S0024-A) and TIA-856-B (3GPP2 C.S0024-B), each of which is hereby incorporated herein in its entirety. The CDMA2000® spread spectrum standard, TIA-2000.2-D (3GPP2 C.S0002-D), upon which 1xEV-DO builds, defines the spread-spectrum characteristics of the 1xEV-DO system and is incorporated herein in its entirety. As defined in these standards, each step in the 1xEV-DO connection setup process occurs sequentially. The PTB time is therefore the sum of the time that each step takes to process.
The current 1xEV-DO connection setup protocol is illustrated in FIGS. 6 and 7, and comprises a series of at least seven sequential steps. FIG. 6 is a timing diagram that illustrates the messaging flows and associated delays inherent in the connection setup. FIG. 7 is a flowchart illustrating the connection setup process at a more conceptual level. It should be appreciated that although only one access network sector 250 is illustrated in FIG. 6, there may by any number of access network sectors 250 according to the topology of the network. Access network sectors 250 are controlled from base stations, each base station typically controlling three access network sectors 250. Connection setup begins at step 710 when the access network's radio node controller 240 receives a connection request (CR) from an access terminal 200 along with a route update message listing the access network sectors that the access terminal is currently monitoring. This list of access network sectors is one of several maintained by the access terminal 200, which divides all of the access network sectors into four groups according to criteria such as signal strength and drop rates: an active set, a candidate set, a neighbor set, and a remaining set. The active set is the set of pilot signals associated with the access network sectors whose control channels are currently being monitored by the access terminal. The active set includes information such as the pseudo-noise (PN) codes used by each of the access network sectors in the active set, as well as an assessment by the access terminal 200 of whether it is desirable to keep each active set sector in the active set. In setting up an initial connection, the route update message that is sent as part of the transmission that occurs at step 710 identifies the active set by listing the PN codes for the sectors that make up the active set, and indicating those that it wishes to keep in the set. At step 720, the access network's radio node controller 240 uses this route update message to update its own active set, keeping only those access network sectors which the access terminal 200 wishes to keep, and which have enough resources that a connection between the access network's radio node controller 240 and access terminal 200 through the given access network sector is possible. The access network's active set may therefore be of smaller or equal size than the active set received from the access terminal 200 at step 710. At step 730 the access network's radio node controller 240 transmits a traffic channel allocation (TCA) message to each of the sectors in its active set. The traffic channel allocation message contains a list of each sector's PN code as well as the data rate control (DRC) covers and data source control (DSC) values for each sector in the active set. Upon receipt of the traffic channel allocation message, the access terminal 200 updates its active set to the initial subset of access network sectors identified in the traffic channel allocation message. At step 740, the access terminal 200 identifies a single access network sector in its active set as its desired serving sector, usually based on its superior signal strength, and begins transmitting its serving sector's DRC cover and DSC value to the access network's radio node controller 240 over a DRC channel. It is said that the access terminal 200 has pointed its DRC at the desired serving sector. The access terminal's transmissions over the DRC channel comprise frames, and each frame comprises sixteen slots, each slot having a duration of approximately 1.667 ms and containing a DRC cover. At step 750 the desired access network sector at which the access terminal 200 is pointing its DRC demodulates enough frames from the signal sent by the access terminal 200 at step 740 that it is able to fully acquire it. This step of acquiring the DRC/DSC information is accomplished using status-filters whose characteristics are specific to the particular DRC as defined in the 1xEV-DO standard. Attaining lock-status for a given serving sector, such that the access terminal's signal is said to be fully acquired, can take up to 200 ms, and represents a significant portion of the PTB delay. One reason the acquisition process takes so long is due to the quality of service required for data-only traffic. Typically, a large number (e.g. 200 or more) of DRC channel slots are demodulated in order to be able to ascertain the desired serving sector with a high level of confidence. Thus, if a confidence threshold of 60% is desired, even if every DRC slot is successfully demodulated, the system will wait until 120 slots out of the first 200 have been successfully demodulated before it determines that it has fully acquired the signal, and because each of those 120 slots is approximately 1.667 ms long, the full acquisition process takes approximately 200 ms. Although lower confidence thresholds are known, confidence thresholds below 40% are considered low, and thresholds higher than 60% are common, leading to even longer PTB times. After this threshold number of DRC channel slots have been demodulated, each indicating the same desired serving sector, the access terminal's signal can be considered fully acquired. At step 755, once the access terminal's DRC/DSC channel has been fully acquired, the access network's radio node controller 240 then sends a reverse traffic channel acknowledgement (RTCACK) indication to the access terminal 200 through its serving sector. In most embodiments of PTT over 1xEV-DO, this is the point at which the “beep” occurs, and is the point in time against which the PTB delay is measured. At step 760, the access terminal 200 sends a traffic channel complete (TCC) indication to the access network's radio node controller 240. Once the connection is completed at step 760, user traffic begins at step 770, and the process enters a continuous phase wherein the connection is regularly updated to maintain the connection as may be required by changes in network conditions and the location of the access terminal 200.
The advantage of this serial process described above is that a high degree of reliability is achieved in setting up 1xEV-DO connections, particularly as far as the full acquisition of the DRC/DSC channel is concerned. This is useful for data transmissions, which can not tolerate errors, and can even conserve bandwidth by reducing the number of retransmissions of data frames. However, this reliability is achieved at the price of speed. Since each step is carried out after the previous step is complete, the PTB delay is the sum of the time that each of the steps takes. As a result, the PTB time in 1xEV-DO systems is a noticeable and undesirable delay. In the case of voice transmissions such as PTT voice transmissions, we have realized that it is more desirable to achieve faster connection speeds than it is to achieve high quality of service during the initial frames of a given voice transmission. In the case of push to talk, we have realized that a faster connection setup method can be achieved by performing in parallel some of the steps previously performed in series.